This invention relates to internally enhanced heat exchange tubes. More particularly, the surface area of the tube bore is increased by either increasing the height of internal fins or reducing the apex angle of internal fins increasing fin density.
In certain refrigeration applications, a heat exchange unit has a liquid refrigerant flowing within a tube while a fluid to be cooled flows externally over the tube. Liquid refrigerants such as trichloromonofluoromethane or dichlorodifluoromethane pass through the exchange tube. The liquid refrigerant absorbs heat from the external liquid and changes state to a gas. The gas phase refrigerant is returned to a compressor, compressed back to liquid, and returned to the heat exchange tube for another cycle.
Some heat exchange tubes have a smooth bore. However, the efficiency of the cooling apparatus is improved when the surface area of the bore is increased. One method for increasing the surface area is to texture the inside wall of the tube.
One method of texturing the bore is to draw a smooth walled tube over a textured plug. The plug deforms the internal bore forming a plurality of parallel spiral ridges. The spiral ridges both increase the surface area and create a controlled flow of refrigerant maximizing the liquid phase contact with the tube.
Both the size of the internal enhancement and the apex angle of the enhancement relative to the tube wall are limited by the method of manufacture. U.S. Pat. No. 4,658,892 to Shinohara et al, discloses that apex angles less than 30.degree. have poor workability and are not practically manufactured. The same patent suggests a fin height of 0.15-0.20 millimeters.
It is known that the maximum efficiency of heat transfer occurs when the ratio of fin height (F.sub.H) to inside diameter (ID) of the heat exchange tube is on the order of 0.02 to 0.03. When the ratio exceeds 0.03, the heat transfer fluid within the tube begins to convert from laminar flow to turbulent flow reducing the flow rate and the corresponding heat transfer rate.
With a fin height limited to 0.15 mm-0.20 mm, the maximum inside diameter of the tube is limited to about: EQU F.sub.H /ID =0.02 EQU 0.2 mm/ID=0.02 EQU ID=10 mm (0.39 in.)
The limit on the inside diameter of the heat exchange tube is a direct result of the method of manufacture. If an alternative method of manufacture could produce higher fins without tearing or breakage, correspondingly larger inside diameter tubes could be made.
One alternative method to manufacture internally or externally enhanced heat exchange tubes is disclosed in U.S. Pat. No. 3,906,605 to McLain which is incorporated in its entirety by reference herein. The patent discloses texturing a metallic strip by passing the strip through textured rolls. The strip is then deformed into a generally circular configuration bringing the edges in close proximity for welding.
The efficiency of a heat exchange tube would be increased if the McLain process could be adapted to produce internally enhanced tubes having either higher internal fins, a smaller apex angle, or both.